The present invention relates generally to fluxless processes for removing surface oxides. More specifically, the invention relates to an apparatus and method comprising same for fluxless reflow and soldering involving electron attachment and field emission at ambient (e.g., non vacuum) pressure.
Reflow and soldering are important processing steps in the assembly of electronic components for making solder joints. The term “reflow” as used herein refers to a process for making a previously applied solder on a substrate melt and flow upon application of an energy source such as, for example, thermal energy. The term “soldering” as used herein refers to a process that allows a melted solder to join at least two metallic substrates. A variety of different reflow and soldering processes may be used in the assembly of electronic devices, such as, but not limited to, reflow of solder bumps used for wafer bumping, reflow soldering used in the assembly of surface-mount electronic components, and wave soldering used in the assembly of insertion-mount components.
Reflow soldering is a process used for outer lead bonding of surface-mount components wherein a chip is transferred with leads in place to the next-level surface mount packages. In the reflow soldering process, components are mounted on the corresponding trace area of a circuit board with a solder paste previously printed on the circuit board. Such formed soldering parts are then loaded into a reflow furnace and passed through heating and cooling zones. Solder joints between component leads and solder lands on the circuit board are formed by melting, wetting, and solidifying the solder paste. To ensure a good wetting of the molten solder on the joining surfaces, organic fluxes are normally contained in the solder pastes to remove initial surface oxides on both solder and base metal and to keep the surfaces in a clean state before solidification. The fluxes are mostly evaporated into vapor phase during soldering; however, the flux volatiles may cause problems, such as forming voids in the solder joints and contaminating the reflow furnace. After soldering, flux residues still remain on the circuit board that can cause corrosion and electric shorts.
Wave soldering is also used for outer lead bonding such as for assembling traditional insertion mount components. It also can be used for surface-mount components by temporarily bonding the components on the circuit board by an adhesive before soldering. For both cases, the circuit boards with components inserted or temporarily bonded have to be cleaned by using a liquid flux to remove oxides on the component leads and solder lands and then passed through a high temperature molten solder bath. The molten solder automatically wets the metal surfaces to be soldered and solder joints are thus formed. The molten solder in the bath has a high tendency to be oxidized, forming solder dross. Therefore the surface of the solder bath has to be frequently cleaned by mechanically removing the dross, which increases the operation cost and the consumption of the solder. After soldering, flux residues remain on the circuit boards, which brings the same problems as described herein for reflow soldering.
Wafer bumping is a process used to make thick metal bumps on the chip bond pads for inner lead bonding. The bumps are commonly made by depositing a solder on the pads and then reflowing (referred to herein as a first reflow) to conduct alloying and to change the shape of the solder bump from a mushroom-shape into a hemispherical-shape. The chip with the first-reflowed bumps is “flipped” to correspond to the footprint of the solder wettable terminals on the substrate and then subjected to a second reflow to form solder joints. These solder joints are referred to herein as inner lead bonds. High-melting point solders (e.g., >300° C.) are normally used in the wafer bumping process because it allows for subsequent assembly steps such as outer lead bonding to proceed using lower-melting point solders (e.g., <230° C.) without disruption of the inner lead bonds.
The shape of the solder bumps after the first reflow is critical. For example, a large bump height is preferable for better bonding and higher fatigue resistance. Further, the bumps formed should preferably be substantially uniform to ensure planarity. Substantially uniform solder bumps having relatively larger bump heights is believed to be associated with an oxide-free bump surface during the first reflow. One approach for removing solder oxides during the first reflow of the solder bumped wafer is applying organic fluxes over the deposited solder bumps, or within a solder paste mixture that has been printed onto the wafer to form the bumps, and reflowing the bumps in an inert environment so that the fluxes can effectively remove initial oxides on the solder surface. However, this approach has its drawbacks. Small voids may form in the solder bumps due to flux decomposition. These voids may not only degrade the electrical and mechanical properties of the formed solder bonds but also destroy the co-planarity of the solder bumped wafer and affect the subsequent chip bonding process. The decomposed flux volatiles can also contaminant the reflow furnace which can increase the maintenance cost. In addition, flux residues are oftentimes left upon the wafer which can cause corrosion and degrade the performance of the assembly.
To remove the flux residues from the reflow and soldering processes described above, a post cleaning process may be adopted using chlorofluorcarbons (CFCs) as cleaning agents. However, post-cleaning adds an additional process step and increases the manufacturing processing time. Further, the use of chlorofluorocarbons (CFCs) as cleaning agents is banned due to the potential damage to the earth's protective ozone layer. Although no-clean fluxes have been developed by using a small amount of activators to reduce residues, there is a trade-off between the gain and loss in the amount of flux residues and the activity of the fluxes.
A good solution to all the problems described above, including void formation, flux volatiles, flux residues, and dross formation, is using a reducing gas as a reflow and soldering environment to replace organic fluxes for removing metal oxides. Such reflow and soldering techniques are called “fluxless reflow” and “fluxless soldering”. Among various fluxless reflow and soldering methods, the use of hydrogen as a reactive gas to reduce oxides on base metals and solders is especially attractive because it is a very clean process (the only by-product is water which can be easily ventilated out of the furnace), and it can be compatible with an open and continued soldering production line (H2 is non-toxic and has a flammable range of 4 to 75%). Therefore, hydrogen fluxless soldering has been a technical goal for a long time.
One previously used hydrogen fluxless method for inner lead bonding has been to employ pure hydrogen for reflow of the solder bumped wafer at temperatures ranging from 400 to 450° C. However, the flammable nature of the pure hydrogen largely limits its application. For soldering processes used in outer lead bonding, such as reflow soldering and wave soldering, the major limitation of using hydrogen to reduce surface oxides is the inefficient and slow reduction rate of metal oxides at the normal processing temperature ranges (<250° C.), especially for solder oxides, which have higher metal-oxygen bond strengths than that of the oxides on the base metals to be soldered. This inefficiency of hydrogen is attributed to the lack of reactivity of the hydrogen molecule at low temperatures. Highly reactive radicals, such as mono-atomic hydrogen, form at temperatures much higher than the normal reflow soldering and wave soldering temperature range. For example, the effective temperature range for pure H2 to reduce tin oxides on a tin-based solder is above 350° C. Such high temperatures may either damage or cause reliability problems to the packed electronic components. Therefore, a catalytic method to assist generating highly reactive H2 radicals, and thus reducing the effective ranges of hydrogen concentration and processing temperature for reducing surface oxides, has been sought by the industry.
Fluxless (dry) soldering has been performed in the prior art using several techniques. One technique is to employ lasers to ablate or heat metal oxides to their vaporization temperatures. Such processes are typically performed under inert or reducing atmospheres to prevent re-oxidation by the released contaminants. However, the melting or boiling points of the oxide and base metal can be similar and it may not be desirable to melt or vaporize the base metal. Therefore, such laser processes are difficult to implement. Lasers are typically expensive and inefficient to operate and require a direct line of sight to the oxide layer. These factors limit the usefulness of laser techniques for most soldering applications.
Surface oxides can be chemically reduced (e.g., to H2O) through exposure to reactive gases (e.g., H2) at elevated temperatures. A mixture containing 5% or greater reducing gas in an inert carrier (e.g., N2) is typically used. The reaction products (e.g., H2O) are then released from the surface by desorption at the elevated temperature and carried away in the gas flow field. Typical process temperatures exceed 350° C. However, this process can be slow and ineffective, even at elevated temperatures.
The speed and effectiveness of the reduction process can be increased using more active reducing species. Such active species can be produced using conventional plasma techniques.
Gas plasmas at audio, radio, or microwave frequencies can be used to produce reactive radicals for surface de-oxidation. In such processes, high intensity electromagnetic radiation is used to ionize and dissociate H2, O2, SF6, or other species, including fluorine-containing compounds, into highly reactive radicals. Surface treatment can be performed at temperatures below 300° C. However, in order to obtain optimum conditions for plasma formation, such processes are typically performed under vacuum conditions. Vacuum operations require expensive equipment and must be performed as a slow, batch process rather than a faster, continuous process. Also, plasmas are typically dispersed diffusely within the process chamber and are difficult to direct at a specific area. Therefore, the reactive species cannot be efficiently utilized in the process. Plasmas can also cause damage to process chambers through a sputtering process, and can produce an accumulation of space charge on dielectric surfaces, leading to possible micro-circuit damage. Microwaves themselves can also cause micro-circuit damage, and substrate or component temperature may be difficult to control during treatment. Plasmas can also release potentially dangerous ultraviolet light. Such processes also require expensive electrical equipment and consume considerable power, thereby reducing their overall cost effectiveness.
U.S. Pat. No. 5,409,543 discloses a process for producing a reactive hydrogen species (i.e., atomic hydrogen) using a hot filament to thermally dissociate molecular hydrogen in a vacuum condition. The energized hydrogen chemically reduces the substrate surface. The temperature of the hot filament may range from 500° C. to 2200° C. Electrically biased grids are used to deflect or capture excess free electrons emitted from the hot filament. The reactive species or atomic hydrogen are produced from mixtures containing 2% to 100% hydrogen in an inert carrier gas.
U.S. Pat. No. 6,203,637 discloses a process for activating hydrogen using the discharge from a thermionic cathode. Electrons emitted from the thermionic cathode create a gas phase discharge which generates active species. The emission process is performed in a separate or remote chamber containing a heated filament. Ions and activated neutrals flow into the treatment chamber to chemically reduce the oxidized metal surface. However, such hot cathode processes require vacuum conditions for optimum effectiveness and filament life. Vacuum operations require expensive equipment, which must be incorporated into soldering conveyor belt systems, thereby reducing their overall cost effectiveness.
Potier, et al., “Fluxless Soldering Under Activated Atmosphere at Ambient Pressure”, Surface Mount International Conference, 1995, San Jose, Calif., and U.S. Pat. Nos. 6,146,503, 6,089,445, 6,021,940, 6,007,637, 5,941,448, 5,858,312 and 5,722,581 describe processes for producing activated H2 (or other reducing gases, such as CH4 or NH3) using electrical discharge. The reducing gas is generally present at “percent levels” in an inert carrier gas (N2). The discharge is produced using an alternating voltage source of “several kilovolts”. Electrons emitted from electrodes in a remote chamber produce exited or unstable species that are substantially free of electrically charged species which are then flowed to the substrate. The resulting processes reduce oxides on the base metal to be soldered at temperatures near 150° C. However, such remote discharge chambers require significant equipment costs and are not easily retrofitted to existing soldering conveyor belt systems. In addition, these processes are typically employed for pre-treating the metal surface before soldering rather than removing solder oxides.
U.S. Pat. No. 5,433,820 describes a surface treatment process using electrical discharge or plasma at atmospheric pressure from a high voltage (1 kV to 50 kV) electrode. The electrode is placed in the proximity of the substrate rather than in a remote chamber. The free electrons emitted from the electrodes produce reactive hydrogen radicals—a plasma containing atomic hydrogen—which then pass through openings in a dielectric shield placed over the oxidized substrate. The dielectric shield concentrates the active hydrogen onto those specific surface locations requiring de-oxidation. However, such dielectric shields can accumulate surface charge that may alter the electric field and inhibit precise process control. The described process is only used to flux base metal surfaces.
Accordingly, there is a problem in the art in providing an economical and efficient process for removing metal oxides from at least one component and/or solder surface under relatively low temperatures to avoid damage to the electronic components. There is a problem in the art in providing a process and apparatus for fluxless soldering under near ambient or atmospheric pressure conditions that avoids the expense of purchasing and maintaining vacuum equipment such as required, for example, in a plasma-based process and apparatus. There is problem in providing a fluxless soldering process and apparatus for internal or non-exposed surfaces of a substrate. Additionally, there is an additional problem in the art for providing a fluxless soldering process that does not involve one or more flammable gases.